1. Field of the Invention
The present invention relates to an automated tracer system for measuring the residence time of wood pulp as it moves through various stages of processing in a pulp mill. The invention comprises a tracer compound for introduction into a pulp slurry, an automated detector for detecting the presence of the tracer compound in the pulp slurry, a controller for relaying a voltage signal from the detector to a processor, and a processor for converting the voltage signal to a calibration curve to determine the amount of tracer detected in the pulp slurry. The present invention also relates to methods for measuring the residence time of pulp in a pulp slurry using the foregoing automated tracer system. The present invention will enable pulp mill personnel to determine conveniently, economically and more accurately than has heretofore been possible, the residence time distribution of pulp as it moves through the various pulp processing steps at a pulp mill. This, in turn, will enable pulp mill personnel to better control the operating conditions under which the pulp is processed and thereby ensure a higher quality final product.
2. Brief Description of the Prior Art
The modern production of most paper pulp is primarily based on two chemical processes: pulping and bleaching.
Pulping involves a combination of chemical impregnation, heat and force to break apart wood fibers into pulp. The pulping operation uses mechanical debarking and grinding of wood chips, followed by cooking in chemical liquor. In chemical pulping, the cooking is carried out in a large tower called a digester. The chemical liquor can be alkali (as in the Kraft process), acidic (as in the sulfite process) or neutral (as in the chemi-thermo-mechanical process).
Bleaching uses oxidizing chemicals to remove some or all of the lignin from the pulp, increasing its brightness and strength and improving such properties as absorption of printing ink, opacity, etc. Bleaching takes place in stages entailing the sequential use of from 1 to about 7 different chemicals, depending on the desired properties of the final pulps. The various bleaching stages are well known to those skilled in the pulp bleaching art and are sometimes referred to by reference to the chemical reactors used to carry out the reactions of chemicals with pulp. Commonly used chemicals in bleaching stages are oxygen, chlorine, chlorine dioxide, sodium hydroxide, hydrogen peroxide, sodium hydrosulfite, and xylanase enzymes.
Like all chemical processes, pulping and bleaching have key variables that must be controlled to obtain efficient use of chemicals and an acceptable final product. The key variables in pulping and bleaching are temperature, pH, chemical concentration, and reaction time.
The method used in most pulping and bleaching processes to carry out the reactions involves pumping the pulp continuously through towers. Typically, the pulp slurries are comprised of 1.5% to 30% pulp solids in aqueous liquor and are pumped through pipes and into or out of the towers. The processes are carried out by adding chemicals to the pulp slurry at or within the entrance to the towers. The pulp then flows through the towers, either upward or downward depending on the process and the mill. The towers hold the pulp within a closed environment, which is beneficial for controlling pH, temperature, and especially chemical concentration. For example, in the case of enzyme treatment of Kraft pulp to enhance bleaching, the process is usually carried out by flowing the pulp down a tower as a slurry of about 8% solids in water at 50.degree. C., and a pH of 7.5. This is accomplished in an enzyme tower that might require about one hour for the pulp to traverse, given the pulp production rate in the mill. For a modern mill that produces 1000 tonnes of pulp per day, this might be a tower 30 meters tall and 8 meters in diameter that is about 1/3 full.
In contrast to pulp pH, temperature, and chemical concentration, which are carefully monitored and controlled by on-line instrumentation, pulp retention time is not measured routinely. The reasons for this neglect relate to the expense, inconvenience and inaccuracy presently encountered by pulp mill operators when employing the prior art detection methods that are now commercially available. Because of these problems, pulp retention time is frequently inferred (i.e., calculated) rather than measured based on the amount of pulp in the tower, the pulp throughput, and by assuming that the pulp flows as a uniform plug through the tower; using a technique known as "plug flow". In the above example, the tower contains 50.2 tonnes of pulp (at 8% solids consistency); at a production rate of 1000 tonnes per day, the retention time for ideal plug flow is 1.2 hours.
A "plug flow retention time" is often quoted because it is readily calculated. However, that time value is often inaccurate because pulp does not generally travel through a tower uniformly as a plug. Rather, the pulp moves more quickly as a core through the center of the tower, by a phenomenon known as "channeling". Pulp channeling was described in detail by Bodenheimer, Channeling in Bleach Towers and Friction Losses in Pulp Stock Lines, Southern Pulp and Paper Manufacturer, September 1969, pp. 42-46 (hereafter "Bodenheimer"). In the example of pulp treated with enzymes, a loss of actual retention time due to channeling causes undertreating of the pulp, which in turn causes inferior bleaching of the pulp thereafter.
Channeling is difficult to observe directly because the inside of the tower is not usually open to view. Bodenheimer reported that the tendency for pulp to channel and the speed with which the pulp traverses the tower is influenced by the tower geometry, pulp level in the tower, wood species, solids consistency, temperature, and pH of the pulp. Of these, only the pulp level can be changed arbitrarily day-to-day, so many mills characterize the retention time at a given tower pulp level and try to maintain that level.
It is known to use tracer systems in the pulp processing art to measure true retention time in a tower. It also is known to use a chemical compound as a tracer to measure retention time by adding it to a pulp slurry of at least about 1.5% solids consistency at the entrance of the tower as a sudden "spike" and to monitor the breakthrough of tracer at the tower outlet. At or above about 1.5% solids consistency, the tracer travels with the pulp and does not migrate significantly into the free liquid continuous phase. Lithium Chloride is the most commonly-used tracer system in the pulp industry. Perkins, Channeling in Continuous Bleaching Cells (Pulp Behavior Patterns in Bleach Towers), January 1971, pp. 191-98 (hereafter "Perkins") describes the use of lithium chloride as a tracer compound, which is measured by an atomic absorption analysis on the pulp slurry. The use of lithium chloride as a tracer compound, although it is the standard method in the industry, suffers from some significant disadvantages--it is expensive to use (costing approximately $5,000 per tracer test) and the analysis is time consuming. Dence and Annergren, in Chapter 3 "Chlorination", p. 62 in The Bleaching of Pulp, R. Singh, Ed., Tappi Press, 1979 also suggest use of trace metals, such as lithium, as a tracer compound, which also are to be detected by employing atomic absorption techniques.
Metcalfe and Eddy, Wastewater Engineering, McGraw-Hill, 1991, p. 1214-1216 recommend sodium chloride and dyes as tracer compounds for a range of systems other than pulp. Unfortunately, sodium chloride has the disadvantage that it is already present in pulp slurry in concentrations of about 50 ppm, not counting that already associated with the chlorine in the bleach, which is of a much higher concentration. Because of this fact, the quantity of sodium chloride required for use in a tracer test is inconveniently large and difficult to handle at the paper mill. Such dyes also discolor the pulp, which is unacceptable to the pulp customer, and can be destroyed by residual levels of bleaching chemicals present in a pulp mill.
Lee et al. (U.S. Pat. No. 4,946,555) ("Lee et al.") describe the use of an inert tracer gas (helium) in a pulp and paper mill in order to determine the utilization of oxygen by an aqueous cellulosic pulp. Although helium may be useful in monitoring the flow of gaseous chemicals inside of a tower, helium gas does not become impregnated into the pulp and thus cannot be used to derive the retention time of the pulp. In addition, helium tracer is not at all useful in downflow towers because of its buoyancy.
Most tracer tests today are carried out with manual sampling at the tower exit and off-line analysis of the tracer. In manual sampling, samples are typically collected periodically (often as frequently as every five minutes) at the tower exit after the introduction of tracer compound into the tower. This method allows pulp mill operators to determine the length of time required for the breakthrough of tracer at the tower exit. However, because the samples are taken at discrete intervals, there is the real possibility that some tracer breakthroughs may not be detected. Moreover, because of the effort and expense involved, tracer tests using manual sampling are typically not carried out more than once every 6 months.
An alternative to manual sampling is on-line analysis of the tracer concentration. While on-line analysis overcomes some the drawbacks in manual sampling, the limited experience in pulp mills using on-line tracer testing has not been successful. Perkins described the use of radioactive tracers and detection by a Geiger counter. This method is now generally considered unsuitable because radioactive tracers are potentially hazardous and their use is frequently restricted by government regulations.